1. Field of the Invention
The present invention relates to an apparatus that eliminates the laser in an Optical Network Unit (ONU) in a Wave Division Multiplexed (WDM) Passive Optical Network (PON).
2. Description of the Related Art
Optical networks have become a standard technology for the transport of information in the telecommunications industry. A number of different optical network standards have been defined, with each having advantages and disadvantages for different uses. Synchronous optical network (SONET) is one standard for optical telecommunications transport. SONET is often used for long-haul, metro level, and access transport applications.
Another standard for optical telecommunications transport is passive optical networks (PONs). PONs are commonly used to address the last mile of the communications infrastructure between the service provider's central office, head end, or point of presence (POP) and business or residential customer locations. Also known as the access network or local loop, the last mile consists predominantly, in residential areas, of copper telephone wires or coaxial cable television (CATV) cables. In metropolitan areas, where there is a high concentration of business customers, the access network often includes high-capacity synchronous optical network (SONET) rings, optical T3 lines, and copper-based T1s.
Bandwidth is increasing dramatically on long-haul networks through the use of wavelength division multiplexing (WDM) and other new technologies. Recently, WDM technology has even begun to penetrate metropolitan-area networks (MAN), boosting their capacity dramatically. At the same time, enterprise local-area networks (LAN) have moved from 10 Mbps to 100 Mbps, and soon many LANs will be upgraded to gigabit Ethernet speeds. The result is a growing gulf between the capacity of metro networks on one side and end-user needs on the other, with the last-mile bottleneck in between.
PONs are one solution to this problem in an attempt to break the last-mile bandwidth bottleneck that other access network technologies do not adequately and economically address.
Important parts of the PON architecture are the Optical Network Unit (ONU) and the Optical Line Termination (OLT), which are active network elements located at end points of a PON. The OLT provides an interface for data to be transmitted over the PON. The ONU provides an interface between the customer's data, video, and telephony networks and the PON. The primary function of the ONU is to receive traffic in an optical format and convert it to the customer's desired format. Many PONs use wavelength division multiplexing (WDM) of multiple signals over each optical fiber. WDM PON provides dedicated optical wavelengths in each direction, for each ONU. This provides improved operations over other types of PON, where the same wavelength(s) are shared by up to 32 (or more) ONU's. However, a typical implementation of WDM PON requires a tuned narrowband laser in the ONU, and a fixed narrowband laser in the OLT dedicated to each ONU. This results in too costly an implementation for access applications. Most PON's today aren't based on WDM PON due to cost, they are APON, EPON, etc where ONU's share wavelengths in both directions. Thus, a need arises for a technique that can both eliminate tuned lasers in the ONU's and also provide shared optical carrier sources for the OLT's.